Vehicles such as automobiles are equipped with a hydraulic controller which controls actuators for the transmission, the clutch, etc. and also provides pressure necessary for the lubrication of these parts. Many of such hydraulic controllers include an air- or water-cooled oil cooler, which cools the hydraulic oil whose temperature increases while it is used for the control and lubrication of the transmission, the clutch, etc.
In such a hydraulic controller, the following method is used commonly. A pressure regulating valve is provided in the circuit of the hydraulic oil to adjust the pressure of the hydraulic oil, and the oil discharged from this pressure regulating valve is returned through an oil cooler to a tank (or a drain pan). In addition to this method, there is another method in which the discharge port of the above mentioned pressure regulating valve is constructed in two ports, one for a line which passes through the oil cooler and the other for a line used for recirculation. In this construction, when the amount of the oil discharged from the pressure regulating valve increases, the oil is directly returned through the recirculation line to the suction port of the pump.
FIG. 6 is a circuit diagram of a conventional lubrication pressure controller which includes a recirculation line. In this device, the hydraulic oil in a tank T is sucked by a pump P through a suction strainer and a suction oil passage 31. After the pressure of the oil is adjusted by two regulator valves 33 and 34 to a predetermined hydraulic pressure (line pressure), the oil is then supplied through a line 35 to a speed change control valve. Excess oil in the pressure adjustment performed by the regulator valves 33 is dumped to a lubrication oil passage 36 (36a and 36b), and the oil in this oil passage is supplied as lubrication oil to the components of the transmission which need lubrication (e.g., to the starting clutch, the planetary gears, the bearing parts of the shafts, etc.). The lubrication oil after being supplied to the speed change mechanism and the other components which require lubrication returns to the tank T through oil passages, for example, passages constituted by the inner walls of the transmission (not shown).
In the lubrication oil passage 36 (between 36a and 36b), which leads the hydraulic oil to the parts which require lubrication, a lubrication valve 71 is provided to adjust the pressure of the oil used for the lubrication. This valve comprises a spool 71a which is biased leftward by a spring 71b. The hydraulic oil moves this spool 71a rightward in correspondence with the magnitude of the pressure supplied into the oil chamber 71c and performs a pressure adjustment which achieves a predetermined lubrication pressure Pb needed for supplying the oil to the parts of the transmission which require lubrication.
For example, when the supply pressure (i.e., the discharge pressure of the regulator valves 33 and 34) in the lubrication oil passage 36a is smaller than the predetermined lubrication pressure Pb, the spool 71a is shifted leftward by the spring 71b, so the hydraulic oil flowing in the lubrication oil passage 36a is partly supplied through a line 80 to the parts requiring lubrication but not through the other lines except the line 36b. On the other hand, if the supply pressure in the lubrication oil passage 36a becomes greater than the predetermined lubrication pressure Pb, the spool 71a is shifted rightward by the hydraulic pressure supplied into the oil chamber 71c, the pressure overpowering the resistance of the spring 71b. In this condition, the line 81a which passes through an oil cooler 85 to the tank T is open together with the line 80, so part of the hydraulic oil is cooled and returned to the tank T. If the supply pressure increases further, then the spool 71a is shifted further rightward (this condition is shown in FIG. 6). In this condition, another line 82 is open together with these lines 80 and 81a. This line 82, which is called "recirculation line", returns directly to the suction oil passage 31 of the pump P without passing through the oil cooler 85 and the tank T.
In vehicular transmissions, it is general that the pump P is directly connected to the output shaft of the engine, so the rotational speed of the pump P is proportional to that of the engine. Thus, there is a tendency that the greater the rotational speed of the engine, the higher the pressure of the hydraulic oil which is discharged from the regulator valves 33 and 34 and supplied into the lubrication oil passage 36a. This means that the operation of the valves described above is performed in correspondence with the rotational speed of the engine and that when the rotational speed of the engine is relatively high, the oil is flown through the recirculation line 82 directly to the suction oil passage 31 of the pump P in addition to the line 81 (81a and 81b) passing through the oil cooler 85.
In addition to this method, which controls the supply of the hydraulic oil to the oil cooler in correspondence with the pressure in the lubrication oil passage by means of the stroke of a valve spool as described above, there is another method in which the oil passage leading to the parts requiring lubrication and the oil passage leading to the oil cooler are switched by a (three-way) electromagnetic valve. This method is disclosed, for example, in Japanese Laid-Open Patent Publication No. H4(1992)-316766. Yet another method, which controls the flow of the hydraulic oil to the oil cooler, is disclosed in Japanese Laid-Open Utility-Model Publication No. H1(1989)-135254. In this method, an electromagnetic valve is provided in the oil passage leading to the oil cooler, and the flow of the hydraulic oil is changed by the on-off control of the valve.
However, in the previously mentioned prior-art lubrication pressure controller (e.g., one shown in FIG. 6), if the lubrication valve 71 is disturbed by an external factor, for example, if the pressure in the lubrication oil passage 36a is disturbed by a drastic change in the amount discharged from the pump or in the control back pressure of the regulator valve 33, and thereby the spool of the lubrication valve experiences an overstroke rightward, then the hydraulic oil flows excessively into the recirculation line 82 and causes a shortage in the amount of the hydraulic oil which is supplied to the lubrication oil passage 36b and to the oil cooler. Furthermore, if the spool is locked in such an overstroke condition, then there is a possibility that the recirculation line 82 will experience a negative pressure by the suction pressure of the pump P and will draw the hydraulic oil into the lubrication oil passage 36a. There is a concern that if this condition occurs, then the shortage of the hydraulic oil which should be supplied to the parts requiring lubrication would continue and result in a lubrication failure.
In the method which switches the oil passage leading to the parts requiring lubrication and the oil passage leading to the oil cooler by a (three-way) electromagnetic valve as disclosed, for example, in Japanese Laid-Open Patent Publication No. H4(1992)-316766, by construction, this switching involves all the hydraulic oil. Thus, the system requires a large electromagnetic valve capable of handling a relatively large flow, and this requirement makes it difficult that this method is to be applied to the condition which requires both the lubrication and the cooling of the transmission.
In the method which is disclosed in Japanese Laid-Open Utility-Model Publication No. H1(1989)-135254, the electromagnetic valve that is provided in the oil passage leading to the oil cooler is used for the on-off control of the valve to change the flow of the hydraulic oil. This control can adjust the flow to the oil cooler only to two levels. Therefore, to balance the flow for the oil cooler with the flow for the recirculation line, another additional valve is necessary.